The present invention relates to induced polarization prospecting and more particularly to improved apparatus and methods for removing noise components from the signals detected in induced polarization prospecting.
The basic methods and apparatus used for induced polarization (IP) mineral prospecting are well known and have been in use for many years. U.S. Pat. No. 4,339,720, issued July 13, 1982, to Halverson, illustrates such a system and is hereby incorporated by reference for all purposes. As discussed in the Halverson patent, numerous improvements have been made in IP systems to allow such systems to be effective at greater search depth. For example, current levels have been increased, electrode spread sizes have been increased, and the frequency of input signals have been reduced. It is generally accepted that the major source of noise in IP prospecting results from naturally occurring earth currents, that is telluric currents, which are detected along with the desired signals.
The improvement taught by Halverson included an electrode arrangement and an exploration method which allowed detection of the telluric noise occurring during the IP prospecting process, where the noise record included very little of the IP signal. This was achieved by using telluric noise detection electrodes spaced apart by a great distance along the prospecting path, at least 1.5 times the maximum search depth of the primary electrode spread. Even with this arrangement some portion of the telluric record was made up of the IP signal. In some cases, the telluric record needs to be corrected for the IP signal before it is in turn used to correct the primary IP records.
Thus, it is seen that it is desirable to provide records of telluric noise with such records being substantially free of induced polarization signal components. It is also desirable that such records be acquired in a simple manner and with the smallest and least complicated electrode spread possible. In addition, it is desirable that orthogonal components of telluric noise be measured so that bends in the prospecting path will not cause degradation of the telluric correction.